This invention relates to solar cell structures and, more particularly, to a solar cell using an amorphous silicon by-pass diode for reverse-bias protection.
A solar cell is formed of two semiconductor layers in facing contact with each other at a semiconductor junction. When illuminated by the sun or otherwise, the solar cell produces a voltage between the semiconductor layers. Advanced solar cells may include more than two semiconductor layers and their respective pairwise semiconductor junctions. The various pairs of semiconductor layers of the advanced solar cells are tuned to the various spectral components of the sun to maximize the power output of the solar cell. The solar cells are made of layers of semiconductor materials that have low ductilities and are relatively brittle.
The voltage and current output of the solar cell are limited by the materials of construction and the surface area of the solar cell. Most commonly, a number of solar cells are electrically interconnected in series and/or parallel arrays to form a solar cell structure that produces higher voltages and/or higher currents than are possible with the single solar cell. Such solar cell structures are now used in both space and terrestrial applications.
The solar cell structure works well when all of the solar cells are illuminated with about the same illumination intensity. However, if one of the solar cells of the solar cell structure is shaded while the others remain fully illuminated, the shaded solar cell is subjected to a reverse-bias condition by the continuing voltage and current output of the remaining solar cells.
Fortunately, each solar cell may be protected against the damage arising during the reverse-bias condition by a parallel by-pass diode that does not pass current when the solar cell is not reverse biased, but passes the impressed current when the solar cell is reverse biased. The by-pass diode thus protects the individual cell against reverse-bias damage.
A number of by-pass diode configurations are in use and are operable, but each has its drawbacks. In one configuration, a discrete by-pass diode is bonded to the back side of the solar cell and interconnected to the semiconductor layers of the solar cell with leads. The discrete by-pass diode protrudes from the back of the solar cell and increases the likelihood that the solar cell will crack during handling or service. In another configuration, the by-pass diode is grown onto the front surface of the solar cell as part of the deposition process and then interconnected to the next cell in series. This approach is complex and causes assembly difficulties as well as reduced production yields and reduced solar cell efficiency. In yet another configuration, the by-pass diode is also grown into the front surface of the solar cell and interconnected with discrete or lithographic techniques. This approach is also complex, and has reduced production yields and reduced solar cell efficiency.
There is a need for an improved approach to the protection of solar cells against reverse-bias damage. The present invention fulfills this need, and further provides related advantages.
The present invention provides a solar cell structure comprising a solar cell protected against reverse-bias damage. The protection utilizes a discrete by-pass diode that is very thin so that it protrudes from the surface of the solar cell only a small distance. Consequently, it is less likely to lead to cracking of the solar cell than are conventional discrete by-pass diodes. The by-pass diode does not protrude from the sides of the solar cell.
A solar cell structure includes a solar cell that produces a voltage when illuminated, and a discrete amorphous silicon by-pass diode. A first by-pass diode terminal of the amorphous silicon by-pass diode is electrically connected to a first side of the active semiconductor structure of the solar cell, and a second by-pass diode terminal of the amorphous silicon by-pass diode is electrically connected to a second side of the active semiconductor structure. Alternatively, the first by-pass diode terminal of the amorphous silicon by-pass diode may be electrically connected to the first side of the active semiconductor structure of the solar cell, and the second by-pass diode terminal of the amorphous silicon by-pass diode may be electrically connected to a second side of the active semiconductor structure of another solar cell.
In accordance with the invention, a solar cell structure comprises a solar cell comprising an active semiconductor structure having a first active semiconductor structure side and a second active semiconductor structure side. There is typically a substrate having a first substrate side in contact with the second active semiconductor structure side and a second substrate side oppositely disposed to the first substrate side. The solar cell produces a voltage between the first active semiconductor structure side and the second active semiconductor structure side when illuminated. The solar cell structure further includes a by-pass diode structure comprising a discrete amorphous silicon by-pass diode, wherein the by-pass diode has a first by-pass diode terminal and a second by-pass diode terminal. A first electrical diode interconnection is in electrical communication between the first active semiconductor structure side and the first by-pass diode terminal. A second electrical diode interconnection is in electrical communication between the second active semiconductor structure side and the second by-pass diode terminal.
Preferably, the first electrical diode interconnection comprises a first-active-semiconductor-structure-side metallization in contact with the first active semiconductor structure side and with the first by-pass diode terminal. The by-pass diode is supported on the first-active-semiconductor-structure-side metallization.
In one embodiment, an electrically semiconductive substrate has a first substrate side comprising a contacting substrate portion that contacts the second active semiconductor structure side, and a noncontacting substrate portion that does not contact the second active semiconductor side. The second electrical diode interconnection is in electrical communication with the noncontacting substrate portion. The second electrical diode interconnection preferably comprises a second-interconnection lead extending between the noncontacting substrate portion and the second by-pass diode terminal.
In another embodiment, the solar cell further comprises a substrate having a first substrate side in contact with the second active semiconductor structure side and a second substrate side oppositely disposed to the first substrate side, and a back-side metallization in contact with the second substrate side. The first electrical diode interconnection comprises a first-active-semiconductor-structure-side metallization in contact with the first-active-semiconductor-structure-side metallization and with the first by-pass diode terminal, and the second electrical diode interconnection is in electrical communication between the back-side metallization and the second by-pass diode terminal. The second electrical diode interconnection desirably is a second-interconnection lead extending between the back-side metallization and the second by-pass diode terminal.
In another embodiment, the solar cell further comprises a substrate having a first substrate side in contact with the second active semiconductor structure side and a second substrate side oppositely disposed to the first substrate side, and a back-side metallization in contact with the second substrate side. The by-pass diode is supported on the back-side metallization. Desirably, the first electrical diode interconnection comprises a first-active-semiconductor-structure-side metallization contacting the first active semiconductor structure side and in electrical communication with the first by-pass diode terminal. The second electrical diode interconnection comprises a back-side metallization in contact with the second substrate side and with the second by-pass diode terminal. The second electrical diode interconnection is preferably a back-side metallization in contact with the second substrate side and with the second by-pass diode terminal.
The present approach may also be utilized by interconnecting two solar cells. In this approach, a solar cell structure comprises a first solar cell comprising a first-cell active semiconductor structure having a first-cell first active semiconductor structure side and a first-cell second active semiconductor structure side, wherein the solar cell produces a voltage between the first-cell first active semiconductor structure side and the first-cell second active semiconductor structure side when illuminated. There is additionally a second solar cell comprising a second-cell active semiconductor structure having a second-cell first active semiconductor structure side and a second-cell second active semiconductor structure side, wherein the solar cell produces a voltage between the second-cell first active semiconductor structure side and the second-cell second active semiconductor structure side when illuminated. The solar cell structure further includes a by-pass diode structure, wherein the by-pass diode structure comprises a discrete amorphous silicon by-pass diode, wherein the by-pass diode has a first by-pass diode terminal and a second by-pass diode terminal, a first electrical diode interconnection in electrical communication between the first-cell first active semiconductor structure side and the first by-pass diode terminal, and a second electrical diode interconnection in electrical communication between the second-cell second active semiconductor structure side and the second by-pass diode terminal.
In a preferred version of this multi-cell approach, the first electrical diode interconnection comprises a first-cell first-active-semiconductor-structure-side metallization in contact with the first-cell first active semiconductor structure side and with the first by-pass diode terminal. The second electrical diode interconnection comprises a second-interconnection lead. The second electrical diode interconnection further comprises a second-cell substrate having a second-cell substrate first side in contact with the second-cell second active semiconductor structure side, and an oppositely disposed second-cell substrate second side, a second-cell back-side metallization in contact with the second-cell substrate second side, and a second-interconnection lead extending between the second-cell back-side metallization and the second by-pass diode terminal.
The present approach thus provides a solar cell structure that uses a thin amorphous silicon by-pass diode for protection against reverse-bias damage. The thin by-pass diode protrudes very little from the surface of the solar cell, reducing the chances that it may be the origin of cracking and failure of the solar cell. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to these preferred embodiments.